Device and method for suppressing signals when inspecting prestressed construction elements

ABSTRACT

A test device and process for inspecting the integrity of prestressed construction elements. The device comprises a testing head having a magnetization device for generating a magnetic field around the construction element. The testing head is connected to a controller device for controlling the magnetization process and for processing signals corresponding to the magnetic field. The testing head magnetizes the construction element over a predetermined measurement section. The controller switches the magnetization device after the magnetization process is completed and then stores and processes the signals recorded.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to a test arrangement for inspecting theintegrity of concrete parts such as, for example the prestressing steelof prestressed concrete construction elements, with a testing head,which has a magnetization device for generating a magnetic field aroundthe construction element, and with a controller for controlling themagnetization process and for processing the signals corresponding withthe magnetic field; as well as to a device for such an inspection.

2. The Prior Art

Fractures of the prestressing reinforcement are detected throughcharacteristic anomalies of the magnetic field surrounding theconstruction element. Such anomalies are based on local variations ofthe magnetization, or of the magnetic permeability of the prestressingreinforcement. The magnetic field is measured in the course of themagnetization process (measurement in the active field), or after themagnetization process (measurement of the residual field). The problemwith the evaluation of the magnetic stray field signals lies in the factthat fracture signals, if any, are superimposed by the signals of theslack reinforcement near the surface.

The test arrangement and the method of magnetic stray field measurementhave been employed for some time now for non-invasive inspection of theintegrity of the prestressing reinforcement of concrete constructionelements. The stray field signals are primarily influenced by the crossgirders. Since the construction element is tested from the surface bia atesting head, the spacing of the cross girders from the testing head issmaller than the spacing of the prestressing reinforcement from thetesting head. The amplitudes of the girders signals are consequentlyhigher than the emplitudes of the fracture signals—if any—in most cases.

When measuring in the active field, fractures of the pre-stressingreinforcement can be detected by the method of measuring at differentmagnetization field intensities. Use is made in this connection of thesaturation behavior of ferromagnetic materials; the reinforcement nearthe surface reaches magnetic saturation earlier than the reinforcementlocated in deeper zones.

Fracture detection thus takes place by this method through thecomparison of signals (correlation analysis; weighted signal difference)of measurements that were carried out at different magnetization fieldintensities. However, the measuring signal still always contains herethe signals of the cross girders.

For suppressing the girder signals in residual field measurements, thecross girders are demagnetized with test arrangements and methods of thestate of the art by controlling the magnetization process in numeroussteps in a manner not described in detail, so that only the signals of alongitudinal reinforcement or prestressing reinforcement will be left.However, it is possible here too that the longitudinal reinforcement isat least partly demagnetized as well, and that possible fracture signalsare therefore not detected.

SUMMARY OF THE INVENTION

Therefore, the problem of the present invention is to provide a testarrangement and a method by which the signals of the cross girders canbe eliminated by way of calculation without having to carry out anyactive demagnetization of the construction element.

The problem is solved in that the testing head magnetizes theconstruction element over a predetermined measurement section in twosuccessive magnetization operations with opposite polarities, and thatthe controller switches off the magnetization device after amagnetization process with one polarity is completed, and stores andprocesses the signals recorded over the measurement section after themagnetization device has been switched off.

The advantage offered by the test arrangement and the method as definedby the invention as compared to active demagnetization of the crossgirders (for example through application of counter fields, oralternating magnetic fields) according to the state of the art is thatthe pole intensity “P” of the magnetization device may remain constantin the magnetization process. This omits costly controlling of the poleintensity of the magnetization device during demagnetization of thegirders, which, under certain circumstances, may influence also themagnetization of the longitudinal reinforcement and particularly of theprestressing reinforcement. On the other hand, only two magnetizationoperations are required as compared to the state of the art describedabove. This means that the inspection expenditure can be substantiallyreduced.

A further advantage consists in that the testing head can move with themagnetization device along the construction element to be inspected,whereby the magnetic field of said magnetization device can be describedin first approximation by the field of a yoke magnet.

A further advantage consists in that the testing head is arranged insuch a way that it is displaceable in directions opposing each other atleast partially in order to generate in this way in the constructionelement magnetizations with different polarities.

A further advantage consists in that the controller superimposes thesignals of the magnetization with different polarities by way ofcalculation, which cancels the signals of the cross girders.

BRIEF DESCRIPTION OF THE DRAWINGS

Other objects and features of the present invention will become apparentfrom the following detailed description considered in connection withthe accompanying drawings. It is to be understood, however, that thedrawings are designed as an illustration only and not as a definition ofthe limit of the invention.

FIG. 1a is a schematic front view of a typical prestressed concretegirder.

FIG. 1b is a schematic side view of the prestressed concrete girdershown in FIG. 1a.

FIG. 2 is a schematic view of a test arrangement with a testing head formeasuring the magnetic stray field according to the present invention.

FIG. 3 is a diagram of the magnetic field Ho of the testing head withthe spacing y=10 cm from the center of the testing head.

FIG. 4 is a diagram of the curve of the stray field of a cross girdermeasured during the forward drive in the active field.

FIG. 5 is a diagram of the curve of the stray field of a cross girder ofa first residual field measurement R1 after the magnetization device hasbeen switched off at an end point of a predetermined measurementsection.

FIG. 6 is a diagram of the curve of the stray field of a cross girder ofa second residual field measurement R2 after the magnetization devicehas been switched off at the starting point of the measurement sectionfrom FIG. 5.

FIG. 7 is the diagram of an averaged signal curve of the residual fieldmeasurements R1 and R2 from FIG. 5 and FIG. 6.

FIG. 8 is a schematic diagram of the sequence of the process as definedby the present invention.

FIG. 9 is a diagram of a first residual field measurement R1 of anexample with application of the test arrangement and process as definedby the present invention.

FIG. 10 is a diagram of a second residual field measurement R2 of theexample from FIG. 9;

FIG. 11 is a diagram of an averaged signal curve of the residual fieldmeasurements R1 and R2 from FIG. 9 and FIG. 10.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT

FIG. 1a shows a front view of the typical structure of a prestressedconcrete girder. A reinforcement is embedded in concrete 3. In detail,said reinforcement is composed of a longitudinal reinforcement 5, crossgirders 7, an envelope tube 9 with prestressing reinforcement 11, and anoverlap joint 13. FIG. 1b shows the prestressed concrete girder fromFIG. 1a by a side view in order to show the arrangement in terms ofspace, which is known per se. The structure of such a prestressedconcrete girder is generally known and therefore not described herein ingreater detail.

FIG. 2 shows a schematic view of the test arrangement as defined by theinvention. A testing head 15 comprises a magnetization device 17 which,in the present case, is an electromagnet with a yoke length “L”. Testinghead 15 is supported on the rollers 19 on a measurement section 21designed in the form of a rail. Length “x” of measurement section 21 ispredetermined. In FIG. 2, a starting point 23 is located at the left endof measurement section 21, and an end point 25 at the right end. Testinghead 15 is arranged and aligned below a construction element 27 to betested. The path of movement of testing head 15 preferably extendsparallel with the surface of the underside of construction element 27.

Two pickup coils 29 and a magnetic field sensor 31 are arranged in theupper region of testing head 15.

Testing head 15 is actively connected to a controller 33. In the presentembodiment, controller 33 is externally connected to testing head 15 viaa line 35. However, it is conceivable also to integrate controller 33 intesting . head 15. Controller 33 is designed in such a way that it bothdrives the testing head 15 and switches the magnetization device 17 onand off. Furthermore, controller 33 is connected to a signal processingunit 35. The signal curves of individual magnetic fields and, in thepresent case, particularly the residual fields are stored in a memoryarea 37 of signal processing unit 35, and called off as required forfurther processing.

The magnetic flow through the construction element is indicated in FIG.2 by a dashed line. The magnetic field of magnetization device 17 isdescribed in a location determined by the coordinates x (length), y(width) and z (height) by the following relations: $\begin{matrix}{{\underset{\_}{H_{0}} = {{- P} \cdot {{grad}\quad\left\lbrack {\frac{1}{r_{1}} - \frac{1}{r_{2}}} \right\rbrack}}}{r_{1,2} = \sqrt{{\left( {{x_{h}m\quad {L/2}} - x} \right)^{2}y^{2}} + z^{2}}}} & (1)\end{matrix}$

P is the pole intensity, L is the length of the yoke, and x_(h) is inthis connection the position of the center of the yoke magnet ormagnetization device 17. Furthermore, it is assumed in equation (1) thatthe poles of the yoke magnet have the coordinates Z_(h)=Y_(h)=0.

FIG. 3 shows the vertical y- and the axial x-component of the magneticfield, whereby the center of the yoke is arranged at x=200 cm, and thevertical spacing from the plane of the pole amounts to 10 cm.

Due to the geometric demagnetization factor of the transverse andlongitudinal reinforcements 7, 5, which have to be viewed aslong-stretched ellipsoids, it can be assumed that the prestressingreinforcement 11 arranged lengthwise relative to the x-axis ismagnetized by the axial field component Hox, whereas the cross girders7, however, are magnetized by the vertical field component Hoy. FIG. 3shows that the vertical field component Hoy is anti-symmetric withrespect to the center of the yoke. When the testing head 15 drives pasta cross girder 7, the magnetism of the latter is reversed provided theintensity of the magnetization field suffices. The sign of the residualfield signal of cross girder 7 therefore is dependent upon whether inthe preceding magnetization operation, the magnetic field of testinghead 15 was switched on only during the forward drive and is switchedoff at end point 25 of measurement section 21 (case R1), or whether themagnetic field is switched on during the forward drive and the returndrive and the magnetization device 17 is switched off at starting point23 of the measurement (case R2 ).

Stray field curves (x-component) calculated and measured in thisconnection are plotted in FIGS. 4 to 7. Said curves are measured by asensor 31 arranged in the center in testing head 15 as it drives past across girder 7. The calculations were carried out with a nonlinearprogram for simulating stray field measurements. The following limitingconditions were selected for the measurements:

Measurement section 21: Start of measurement at xo=0 cm

End of measurement at xl=500 cm

Magnetization device 17 Pole intensity: P=75000 Acm

Yoke length L=50 cm

Cross girder 7 (x=250 cm): Diameter=1 cm

Length=50 cm

Spacing from testing head 15:7.0 cm

It is obvious that the signal of cross girder 7, when measured in theactive field, differs significantly from the form of the signal in theresidual field measurement; however, the amplitudes are in the sameorder of magnitude. Identical signals forms are obtained in the residualfield measurements R1 and R2, but with different signs. The signals ofcross girders 7 almost cancel each other when an average value is formedbased on both measurements. This can be seen especially in FIG. 7.

The important steps of the process sequence for eliminating the signalsof the cross reinforcement according to FIG. 8 are described in detailin the following.

1st Step: Magnetization

Testing head 15 is driven with a constant magnetic field (poe intensityP=Po) switched on, from starting point 23 to end point 25 of measurementsection 21, and switched off there (at the end point)(pole intensityP=0).

2nd Step: Residual field measurement R1(x)

The residual field measurement is carried out at pole intensity P=0 astesting head 15 is returning, or as testing head 15 is driving againforward up to end point 25. Storage of the measured values in dependencyof location x.

3rd Step: Renewed magnetization

Testing head 15 is driven with the magnetic field switched on (poleintensity P=Po) from starting point 23 to end point 25 and back again tostarting point 23 and switched off there (at starting point 23).

4th Step: Residual field measurement R2(x)

A residual field measurement is carried out again at pole intensity P=0during the return drive or while testing head is driving again forward.Storage of the measured values in dependency of location x.

5th Step: Mathematical averaging R(x)

Mathematical superposition by addition of the two measurements R1 and R2on each location coordinate x of measurement section 21:

R(x)=½(R1(x)+R2(x).  (2)

Almost only the signals of the reinforcements arranged along thedirection of displacement are thus still present in dataset R(x).

The process is now demonstrated in the following on a practical example,in which a prestressed concrete girder 1 was inspected. The magneticfield measurement was carried out in this connection with five sensors31, which were arranged next to each other with a spacing of 4 cm inbetween. The residual field measurements R1 and R2 are represented inFIGS. 9 and 10.

The averaged residual field measurement reveals two fracture locationsat x=310 and x=440 cm. The fracture at x=310 is located in the centerand the fracture at 440 cm is located on the edge of girder 1. Theexample clearly shows that it is possible by the method represented hereto detect in the residual field measurement fracture signals oflongitudinal reinforcement 5 which are not visible in the directresidual field measurement (R1 or R2 ).

The important features of the invention are summarized as follows:

1. Controlling of the measuring sequence according to the flow chartaccording to FIG. 8, whereby two residual field measurements are carriedout in which the magnetizations of all reinforcements (cross girders)arranged transversely to the longitudinal reinforcement have oppositepolarities.

2. Determination of the new measuring signal by forming the averagevalue according to equation (2), in which the signals of the crossgirders are largely eliminated and which substantially still containsonly the residual field signal of the reinforcements arrangedlengthwise.

3. Elimination of the residual field signals of the reinforcementsarranged lengthwise by application of the measuring process in measuringdirections extending vertically relative to the first measuringdirection.

According, while only one embodiment of the present invention has beenshown and describe, it is obvious that many changes and modificationsmay be made thereunto without departing from the spirit and scope of theinvention.

What is claimed is:
 1. A test device for suppressing signals wheninspecting a prestressed construction element having a prestressingreinforcement and a transverse stirrup, comprising: a testing head (15)for magnetizing the construction element (27) over a predeterminedmeasurement section (21) in two successive magnetization operations byswitching a testing head (15) in opposite direction of each other foropposite polarities with opposite polarities; a magnetization device(17) disposed on said testing head for generating a magnetic fieldaround the construction element, said magnetizing device being movablebetween a starting point (23) and an end point (25) of a range ofmeasurement, said magnetic field comprising an axial field component(H0x) which magnetizes the prestressing reinforcement and a verticalfield component (H0y) which magnetizes the transverse stirrups (7); acontroller (33) attached to said testing head for controlling themagnetization, wherein said controller switches off the magnetizationdevice (17) at said starting point (23) and at the end point (25); and asignal processing device (35) connected to said controller (33) forperforming a residual field measurement and storing available residualfield signals (R1, R2) of said vertical field component after saidmagnetizing device is switched off, wherein said controllermathematically superimposes said residual field signals (R1, R2) storedafter the magnetizing process.
 2. The test device according to claim 1,wherein said control device (33) generates an output test signal byforming a mean value following said mathematical superimposition of theindividual residual field signals (R1, R2).
 3. The test device accordingto claim 1, wherein said magnetization device (17) magnetizes thetransverse stirrups (7) in said magnetizing processes across drivingsections opposing one another.
 4. The test device according to claim 1,wherein said magnetization device (17) is drivably disposed between saidstarting point (23) and said end point (25) of said measuring range (21)in directions opposing one another.
 5. A method for suppressing signalswhen inspecting a prestressed construction component having a transversestirrup and a prestressing reinforcement, comprising the steps of:generating a magnetic field (H0) between a starting point and an endpoint of a measuring range with a magnetization device, said magneticfield comprising an axial field component (H0x) which magnetizes theprestressing reinforcement, and a vertical field component (H0y) whichmagnetizes the transverse stirrups; switching on said magnetizing deviceat said starting point of said measuring range for a first magnetizingdrive in a first driving direction over said measuring range; switchingoff said magnetizing device at said end point of said measuring range;measuring a first residual field signal (R1) of said vertical fieldcomponent (H0y) of said first magnetizing drive; storing said measuredfirst residual field signals (R1) of the transverse stirrups; switchingon said magnetizing device at said starting point of said measuringrange for a second magnetizing drive in an opposite driving directionover said measuring range; switching off said magnetizing device at whenit returns to said starting point of said measuring range; measuring asecond residual field signal (R2) of said vertical field component (H0y)of said second magnetizing drive; storing said measured second residualfield signals (R2) of the transverse stirrups; and processing saidstored first and second residual field signals (R1) and (R2) after themagnetizing device is switched off wherein a controller mathematicallysuperimposes the residual field signals.